1. Field of the Invention
This invention relates to a radiation sensing system, and particularly to a radiation sensing system using a signal accumulating type radiation sensing device. More specifically, the invention relates to the technique of controlling the signal accumulating time in such radiation sensing system.
2. Description of the Prior Art
In an apparatus using a so-called signal accumulating type radiation sensing device known as CCD, BBD or MOS sensor, where it is desired to obtain an always stable level of sensing output (scanning output) from the sensing device for a radiation whose intensity varies over a very wide range, it is necessary to control the amount of accumulation of the signal of the sensing device to stabilize it in some form or other irrespective of the difference in intensity of the radiation. This can be accomplished, for example, by disposing a diaphragm or a shutter forwardly of the sensing device and controlling the amount of radiation received by the sensing device (this is the same as the control of the exposure amount of the film in a photographic camera) or alternatively by controlling the accumulating time of the signal of the sensing device, namely, the time during which the accumulation of the produced signal is effected in the sensing device. The latter method is advantageous over the former method in that any large-scale apparatus for physically controlling the amount of incident radiation is not necessary.
Various methods of controlling the signal accumulating time of such signal accumulating type radiation sensing device have heretofore been proposed and some of them have been put into practice. For example, a method of presetting an accumulating time corresponding to the then intensity of the radiation prior to the signal accumulation in the sensing device and effecting the signal accumulation under the present accumulating time, and a method of monitoring the amount of accumulated signal during the signal accumulation in the sensing device and stopping the accumulating operation when said amount has reached a predetermined level are known.
In any of the heretofore known control methods, including the above-described two methods, signal accumulation is continuously effected by the sensing device under a controlled time, but this may lead to the following inconveniences.
For example, assuming that the apparatus is used under the illumination by an artificial light source such as a fluorescent lamp driven by an AC power source of 50 Hz (or 60 Hz), the flicker frequency of the illuminating light is 100 Hz (or 120 Hz) and accordingly, the flicker period is 10 msec. (or 50/6 msec.) and in such case, when the signal accumulating time of the sensing device has become, for example, 1 msec., 2 msec. or 3 msec. which is shorter than the flicker period of the illuminating light, if the timing of signal accumulation, i.e., sampling, is not synchronous with the flicker of the illuminating light, the amount of accumulated signal will fluctuate in each signal accumulation. Accordingly, in such a case, in order that the amount of accumulated signal may be constant in each signal accumulation, a new contrivance for controlling the timing of signal accumulation to synchronize it with the flicker of the illuminating light becomes necessary and this leads to the inconvenience that the control circuit becomes large-scale and complicated. Further, in this case, it is seen that if a sampling period fit for both the flicker frequencies 100 Hz and 120 Hz of light sources driven by AC power sources of 50 Hz and 60 Hz, respectively, is calculated, it is an integer times as great as 50 msec., but assuming that such apparatus is applied to a TV camera or a video camera, the field period is 1/60 sec. (field frequency 60 Hz) in the NTSC system adopted in U.S.A. and Japan, for example, and the sampling synchronized with such flicker gives rise to the inconvenience that the field frequency 60 Hz in TV cannot be satisfied.